This invention relates to the field of hydrofining (hydrodesulfurization and/or hydrodenitrogenization) of petroleum fractions and in particular to the hydrofining of fuel distillates. This invention applies in particular to hydrodesulfurization of gas oil fractions for the production of engine gas oils that meet future specifications as regards sulfur. It also applies to the intensive hydrodenitrogenization of fuel distillates, particularly with a view to subsequent hydrocracking of them. In the case of hydrodesulfurization, the sulfur specification will soon be 500 ppm in most countries. In Europe, this specification will be mandatory starting on Oct. 1, 1996. In the near future, the specification will be even more strict; values of 50 ppm of sulfur or even less are often mentioned. In the text below, this invention is described in connection with hydrodesulfurization; however, desulfurization shall not be considered a limitation of this process.
The gas oil fractions being used in the production of engine gas oils may come from various sources; they may be gas oil fractions from direct distillation or gas oil fractions from processes of hydrocarbon conversion, for example, catalytic cracking, hydrocracking, visbreaking, the coking process, or various catalytic residue hydroconversion processes, or it may be a mixture of several of these fractions. These gas oil fractions contain variable amounts of sulfur, ranging typically from 0.2% to 28 weight. To bring the sulfur content of these fractions to the level of future specifications, it is necessary to process them by the catalytic hydrodesulfurization process (HDS).
In the HDS process, the adjustment parameters ordinarily used to adjust the sulfur content are the increase in the temperature of the reactor, the increase in the hydrogen pressure, and the increase in the residue time of the fluids in the reactor. When one or more of these three adjustment parameters is increased, however, there is a tendency, not provided for by kinetic laws, for desulfurization levels to level off when the sulfur level becomes very low. This degradation in desulfurization performance is in particular very significant in the case where the fluids flow from top to bottom in a reactor that works with downward flow. This problem therefore greatly limits the chances for industrial development of such a process using reactors often called "trickle bed" reactors by ones skilled in the art, for the production of engine gas oils with very low sulfur contents.
Without being bound by any particular theory, it seems that this degradation in reactor performance is due, at least partly, to problems in the distribution of fluids and the segregation of the gaseous and liquid phases, which has the effect that a portion of the liquid passes through the reactor along preferential paths (short circuits), therefore desulfurizes poorly and winds up contaminating the product at the output of the reactor.